With the development of downsized optical modules including an optical modulator, for example, and that mutually convert electrical signals and optical signals, there have been increasing demands for downsized optical transmission devices that transmit and receive optical signals. Furthermore, there have also been increasing demands for large-capacity optical transmission devices having a baud rate of 64G baud, for example.
Examples of such optical transmission devices include, but are not limited to, optical transmission devices that perform coherent optical communications. The optical transmission devices that perform coherent optical communications transmit and receive two polarized-wave components of light from a light source with an I-channel signal and a Q-channel signal superimposed thereon, for example. The optical transmission devices have high-speed signal connections of four channels for transmission and four channels for reception, that is, a total of eight channels. In a case where differential signals are used as the I-channel signal and the Q-channel signal, the number of high-speed signal connections is 16 channels, which is twice as many as eight channels.
As described above, the optical transmission devices have high-speed signal connections of a number of channels. Therefore, the optical transmission devices may be downsized by integrating the transmission function and the reception function into a single optical module.
[Patent document 1]: Japanese Laid-open Patent Publication No. 2012-18289
[Patent document 2]: Japanese Laid-open Patent Publication No. 2014-103138
The optical module in which the transmission function and the reception function are integrated has large cross talk between wiring. Specifically, the optical module includes terminals corresponding to eight channels or 16 channels. The wiring connected to the terminals transmits high-speed signals of 32G baud or higher, for example. As a result, a number of terminals and wiring that transmit the high-speed signals are arranged close to each other, thereby causing cross talk.
Power of transmission electrical signals received by the optical module is substantially ten times the magnitude of power of reception electrical signals output from the optical module. In a case where the terminals and the wiring for the transmission electrical signals are arranged close to those for the reception electrical signals, cross talk caused by the transmission electrical signals serves as noise not negligible for the reception electrical signals.